Evolution of secondary phases in austenitic stainless steels during long-term exposures at 600, 650 and 800 deg C

摘要

Three austenitic steels (18Cr-8Ni, 18Cr-10Ni, 21Cr-30Ni), used for long-term applications at temperatures between 600 and 800 deg C were investigated. In the investigation, metallography, transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy, and scanning electron microscopy were used. In additional to the experimental measurements, thermodynamic predictions were done using the ThermoCalc software and the non-commercial database STEEL16F. Various combinations of M_(23)G_6, sigma, and MC phases were identified in the austenite matrix of these steels. It was confirmed experimentally that extra large particles (up to 10 mu m) observed in the 21Cr-30Ni steel are M_(23)G_6, even though this carbide was not predicted as the equilibrium carbide at service temperature (800 deg C). The analytical-experimental approach, combining thermodynamic predictions and experimental measurements, was found to be reliable for the characterization of austenitic steels.